Mobile-First Layout & UI Polish

A number of UX improvements are introduced across the App.

• Drag-and-drop is streamlined: images can be dropped directly onto the "reference image" box in the model-creation flow.

• A responsive grid system is adopted for modals and banners to ensure optimal display on all devices.

• Keyboard navigation is enabled: ←/→ can be used to browse images in Gallery and Model History.

• Scroll position is preserved after image or model deletion, preventing automatic return to the top.

• Empty prompts are supported during model creation; default values are applied to ensure uninterrupted generation.